1. Field of the Invention
The present invention relates to an optical sensing device which employs fiber Bragg grating and includes both a scanning filter and an interferometer which together function as the interrogation and multiplexing system of the sensing device.
2. Description of the Prior Art
The introduction of fiber Bragg gratings (FBG""s) and the development of subsequent techniques with which to mass-produce them has spurred significant research activity in the development of FBG-based sensors and interrogation systems that detect shifts in the center wavelengths of the gratings. The wavelength shifts are caused by axial strains exerted on the fiber in which the gratings are written. This has led to widespread application of gratings in strain gauges (Kersey et al, J. Lightwave Tech., 15, 1442-63,1997; Vohra et al, IEICE Trans. Electron., E83-C, 454-61,2000; Maaskant et al, Cement Concrete Composites, 19, 21-3, 1996; Todd et al, Proc. Int. Modal Analysis Conf. XVIII, San Antonio, Tex., 2000) or, when provided with appropriate transducers, as sensors for other measurands such as acceleration (Todd et al, Photon Technol. Lett., 10, 1605-7,1998), soil pressure (Chang et al, Proc. 14th Int. Conf. on Optical Fiber Sensors, Venice, 2000) and oil well gas pressure (Yamate et al, Proc. 14th Int. Conf. on Optical Fiber Sensors, Venice, 2000). Since several FBG""s can be written into a single fibre, ease of multiplexing as well as an immunity to electromagnetic interference are important features that FBG-based devices offer.
The current state of the art in wavelength shift detection systems is based primarily on variations of four designs: scanning Fabry-Perot (SFP) filter-based interrogation (Kersey et al, Opt. Lett., 18, 1370-2, 1993), tunable acousto-optic filter interrogation (Geiger et al, Electron. Lett. 31, 1006-7, 1995), wavelength division-multiplexed (WDM) interferometric interrogation (Kersey et al, Electron. Lett. 28, 236-7, 1992) and prism/CCD-array techniques (Askins et al, Smart Sensing, Processing and Instrumentation, Proc. SPIE 2444, 257-61, 1995).
The tunable filter techniques (SFP and acousto-optic) use a broadband source and broadband detectors to interrogate multiple FBG""s in series in single array by tuning the filter such that a single grating is illuminated at any given time within the optical bandwidth of the source. A control voltage applied to the SFP filter (or a control frequency applied to the acousto-optic filter) is used to track the reflection peaks of each grating. In the case of the SFP filter, the relationship between the control voltage and the passband wavelength of the SFP filter is not a linear one, and thus curve-fitting calibration techniques must be employed.
The WDM interferometer and CCD-array techniques use a broadband source and broadband detectors in conjunction with wavelength discrimination components to interrogate multiple gratings simultaneously. For the WDM system, the gratings must be centered within the passbands of the WDM filters. Accordingly, large wavelength shifts which place the gratings outside the filter bandwidths cannot be tolerated. For this reason, and because very low-frequency interrogation of the interferometer is difficult due to inherent drift destabilization, the WDM system is often used for high-frequency, low-amplitude measurements. The CCD-array system is much more flexible in its multiplexing capability and can simultaneously sample multiple gratings without confining wavelength shifts to discrete bands. From a system performance perspective, the SFP filter systems tend to have a low sample rate ( less than 1 kHz) due to limitations in the required electronics and a dynamic noise floor of roughly 100 nxcex5Hzxe2x88x921/2, while the interferometer-based systems may have a measurement bandwidth up to tens of kilohertz and a noise floor of 5-10 nxcex5Hzxe2x88x921/2 at 100 Hz, with a rapidly rising noise floor approaching the static end of the frequency spectrum due to active component requirements. Despite lesser bandwidth and sensitivity, the scanning filter designs are frequently employed because of the ease with which both static and quasi-static measurements may be made and because of the ease with which the multiplexing of several gratings ( greater than 10) on a single fiber strand may be achieved.
It is an objective of certain embodiments of the present invention to provide a fiber Bragg grating sensor interrogation system that combines the ease-of-multiplexing capability of an SFP filter-based system and the high-resolution capability of an interferometer-based system.
It is another objective of certain embodiments of the present invention to provide a fiber Bragg grating sensor interrogation system that surpasses the dynamic range of both an SFP filter system and an interferometer-based system and which exceeds the bandwidth limitations of the SFP filter system.
It is a further objective of certain embodiments of the present invention to provide a fiber Bragg grating-based sensing device, which employs a passive technique by utilizing a fiber Bragg grating-based wavelength reference device in order to compensate for interferometer drift and thermal variation in its fiber Bragg gratings such that quasi-static strain measurements can be made.
It is still a further objective of certain embodiments of the present invention to provide a demodulator used in a fiber Bragg grating-based sensor device, which can provide a substantially accurate measurement even when the intensity of the light source, or the decay of the couplers, filters or interferometer in the device is unstable.
It is still a further objective of certain embodiments of the present invention to provide a fiber Bragg grating-based sensing device which has a high resolution, is easy to multiplex, allows use of a large bandwidth, and has a high tolerance for fluctuations in the intensity of its light source and for changes in environmental factors such as temperature.
In a first aspect, the present invention relates to a device for measuring a change in an environment. The device first includes an optical fiber having a plurality of fiber Bragg gratings. Each of the fiber Bragg gratings has a predetermined reflection wavelength. The device further includes a broadband light source optically coupled to the optical fiber; and a scanning filter. The scanning filter is optically coupled to the optical fiber. The device further includes an interferometer optically coupled to the scanning filter; a first photodetector generating a first output, wherein the first photodetector is optically coupled to the interferometer; a second photodetector generating a second output, wherein the second photodetector is optically coupled to the interferometer; and a third photodetector generating a third output, wherein the third photodetector is optically coupled to the interferometer.
In a second aspect, the present invention relates to an interrogating system for a fiber optical sensor having fiber Bragg gratings. The interrogating system includes a scanning filter optically coupled to a fiber optical sensor; an interferometer optically coupled to the scanning filter; a first photodetector optically coupled to the interferometer; a second photodetector optically coupled to the interferometer; and a third photodetector optically coupled to the interferometer.
In a third aspect, the present invention relates to a method for measuring a change in an environment. The method for measuring the change in the environment includes the steps of: a) illuminating a fiber having a plurality of fiber Bragg gratings with different center wavelengths with a broadband light to generate a reflected light from each of the Bragg gratings, wherein each reflected light is reflected from a specific fiber Bragg grating and has a wavelength which is the sum of the center wavelength characteristic of the specific fiber Bragg grating and a wavelength shift characteristic of one or more changes in the environment; b) filtering each reflected light based on its wavelength; c) encoding the wavelength of the each reflected light which has been filtered into a phase signal embedded in at least three interference lights; d) converting each of the at least three interference lights into voltages; e) demodulating the voltages signals to obtain the phase of the phase signal; and f) calculating the wavelength shift of each reflected light from the phase of the phase signal; and g) determining a change in the environment from said wavelength shift.
In a fourth aspect, the present invention relates to a demodulator for processing outputs from an interrogation system which may be used in an optical sensing device having fiber Bragg gratings. The demodulator includes: a) means for determining the tangent of a phase of a phase signal using the outputs from the interrogation system; b) means for determining an arctangent of the tangent of the phase of the phase signal; and c) means for phase-unwrapping the determined arctangent.
In a fifth aspect, the present invention relates to a method for processing outputs from an interrogation system which may be used in an optical sensing device having fiber Bragg gratings. The method includes the steps of: a) determining the tangent of a phase of a phase signal using the outputs from the interrogating system; b) determining an arctangent of the tangent of the phase of the phase signal; and c) phase-unwrapping the determined arctangent.